1. Field of the Invention
This invention relates to wet strength resin compositions for paper and a method for making them.
2. Description of the Related Art
Aminopolyamide-epichlorohydrin resins were the first commercially significant neutral-to-alkaline curing wet strength resins for paper. In addition to the health and safety benefits that result from the resins being formaldehyde-free, their use also affords such benefits as reduced paper-machine corrosion and improved softness and absorbency in treated webs compared to urea-formaldehyde-treated webs. U.S. Pat. No. 2,926,116 teaches high wet strength paper is obtained by treating paper pulp with a water soluble polymeric reaction product of epichlorohydrin and a polyamide derived from a polyalkylenepolyamine and a dicarboxylic acid having from 3 to 10 carbon atoms. Other patents teaching the preparation and/or use of aminopolyamide-epichlorohydrin resins in wet strength applications for paper include U.S. Pat. Nos. 2,926,154; 3,049,469; 3,058,873; 3,066,066; 3,125,552; 3,186,900; 3,197,427; 3,224,986; 3,224,990; 3,227,615; 3,227,671; 3,239,491; 3,240,761; 3,248,280; 3,250,664; 3,311,594; 3,329,657; 3,332,834; 3,332,901; 3,352,833; 3,248,280; 3,442,754; 3,459,697; 3,483,077; 3,609,126; 4,714,736; British patents 1,073,444 and 1,218,394; Finnish patent 36,237 (CA 65: 50543d); French patent 1,522,583 (CA 71: 82835d); German patents 1,906,561 (CA 72: 45235h); 2,938,588 (CA 95: 9046t); 3,323,732 (CA 102: 151160c); Japanese patents 70 27,833 (CA 74: 4182m); 71 08,875 (CA 75: 49990k); 71 12,083 (CA 76: 115106a); 71 12,088 (CA 76: 115107b); 71 36,485 (CA 77: 90336f); Netherlands application 6,410,230 (CA 63: P5858h); South African patent 68 05,823 (CA 71: 114420h); Swedish patent 210,023 (CA 70: 20755y).
One of the drawbacks associated with the use of aminopolyamide-epichlorohydrin wet strength resins is the emission of harmful chlorinated compounds into the water systems of pulp and paper mills. These chlorinated compounds, which are the by-products of the manufacture of the aminopolyamide-epichlorohydrin resins, have been identified as epichlorohydrin, 1,3-dichloro-2-propanol, and 3-chloro-1,2-propanediol. These chlorinated organics are usually discharged into effluent waste water systems from pulp and paper mills because they are only partially substantive to cellulose pulp fibers. Since permissible amounts of halogenated organics in waste waters is ever decreasing, considerable effort has been expended to reduce the amount of these materials in aminopolyamide-epichlorohydrin wet strength resins. For example, European Patent Application 335,158 teaches that wet strength resins having organic chlorine values of about 0.001% are obtained by reaction of an aminopolyamide with a glycidyl sulfonate. European Patent Application 332,967 teaches that wet strength compositions containing 15% by weight of aminopolyamide-epichlorohydrin resins having organic chlorine contents of about 0.16% by weight of total composition or 1.1% by weight of aminopolyamide-epichlorohydrin acid salt resin are obtained by adding epichlorohydrin to an aqueous solution of aminopolyamide at 25.degree. C.-30.degree. C. and then heating the reaction mixture to 75.degree. C. for a period of time sufficient to increase the viscosity of a 15% solids solution to 55 MPa-s at 25.degree. C. The moles of epichlorohydrin/amine equivalents weight (E/N, infra) of the composition is equal to about 1.0. European Patent Application 332,967 teaches an elevated epichlorohydrin reaction temperature of 75.degree. C. as compared to from about 15.degree. C. to about 35.degree. C. according to the invention. The application does not teach or suggest that very low total organically bound chlorine levels are obtainable by lowering the epichlorohydrin reaction temperature while maintaining the E/N ratio in the range of from 0.6 to 1.08. European Patent Application 335,157 teaches an epichlorohydrin reaction temperature of 55.degree. C. as compared to from about 15.degree. C. to about 35.degree. C. according to the invention and an E/N ratio of about 1.2 as compared to maintaining the E/N ratio in the range of from 0.6 to 1.08 according to the invention. The composition produced by the disclosed process contains about 3.12% total organic chlorine based upon the weight of the aminopolyamide-epichlorohydrin acid salt resin. The application does not teach or suggest that very low total organic chlorine levels are obtainable by lowering the epichlorohydrin reaction temperature and lowering the E/N ratio. Wochenblatt fur Papierfabrikation 2, 63-67 (1989) teaches that a modified aminopolyamide-epichlorohydrin resin has an absorbable organic halogens content 70% lower than conventional commercial products. The wet strength resin compositions according to the invention contain organic chlorine compounds in amounts which are at least 99% lower than commercial aminopolyamide-epichlorohydrin resins. Wochenblatt fur Papierfabrikation 2, 63-67 (1989) neither identifies the modified aminopolyamide-epichlorohydrin resin nor teaches how to make it. U.S. Pat. No. 3,887,510 teaches an improvement in the process for the preparation of an aqueous solution of cationic thermosetting resin. The process being improved comprises forming a long-chain water-soluble polyamide from a diester of a C3 to C6 saturated aliphatic dicarboxylic acid with a C1 to C3 saturated aliphatic alcohol and a polyalkylene polyamine having two primary amine groups and at least one secondary amine group the mol ratio of the polyamine to the diester being from about 0.8/1 to 1.4/1, and then reacting the polyamide in aqueous solution with epichlorohydrin in amount such that the mol ratio of epichlorohydrin to the sum of secondary and tertiary amine groups in the polyamide is from about 0.5/1 to about 1.8/1. The improvement comprises forming the polyamide by reacting the polyalkylene polyamine under substantially anhydrous conditions at from about 80 .degree. to about 95.degree. C., from about 1.5 to 3 hours, at atmospheric pressure with the diester and removing the alcohol formed at a temperature no higher than about 105.degree. C. to produce a polyamide having an intrinsic viscosity of from about 0.08 to 0.12 dl/gram in 1 n aqueous ammonium chloride at 25.degree. C. The patent also teaches that removing the alcohol at a temperature of about 120.degree. C. at 140.degree. C. is effective in obtaining products with certain superior combinations of properties and that some benefits can be obtained when the temperature is as high as about 170.degree. C. However, the patent does not teach which properties are affected by the maintaining the temperatures in the stated ranges. U.S. Pat. No. 5,017,642 teaches a process for producing a cationic thermosetting resin containing a very small amount of organic halogen compounds which comprises (i) heating an aliphatic dicarboxylic acid and a polyalkylenepolyamine in a molar ratio of the aliphatic dicarboxylic acid to the polyalkylenepolyamine of 1:1.0 to 1:1.2 until the viscosity at 25.degree. C. of a 50% aqueous solution of resulting polyamidopolyamine reaches 400 to 1000 cps to obtain a polyamidopolyamine; (ii) reacting the polyamidopolyamine with epihalohydrin in an aqueous medium at a temperature from 10.degree. to 55.degree. C. under such conditions that the amount of epihalohydrin is from 0.85 to 1.40 moles per mole of secondary amino group present in the polyamidopolyamine when the reaction temperature is not lower than 10.degree. C. and lower than 45.degree. C. or from 0.85 to 1.25 moles per mole of said secondary amino group when the reaction temperature is from 45.degree. to 55.degree. C., a total concentration of reactants in the aqueous medium is 20 to 70% by weight until a total amount of unreacted epichlorohydrin and other organic halogen compound does not change substantially; (iii) maintaining or lowering the concentration of the resulting aqueous solution of the reaction product; (iv) keeping the temperature of the aqueous solution from 25.degree. to 70.degree. C. until the viscosity at 25.degree. C. of the aqueous solution at a concentration of 15% reaches 10 to 100 cps; (v) adjusting pH of the resulting aqueous solution of the product to 3 to 5 at 25.degree. C. U.S. Pat. No. 5,019,606 teaches that a polyamidoamine-epichlorohydrin wet strength resin wherein the amount of organically bound chlorine in the resin is no more than 4% by weight of the resin is made by a process which comprises reacting a water soluble polyamidoamine with epichlorohydrin in a mole ratio of polyamidoamine to epichlorohydrin of 0.6 to 2.0 and then reacting with a sufficient amount of base at 25.degree. C.-95.degree. C. so that the pH is greater than 8 and subsequently adjusting the pH to not more than 7 by the addition of acid.
The related art does not teach wet strength resins compositions comprised of an aminopolyamide-epichlorohydrin acid salt resin and up to 0.7% by weight total organically bound chlorine based on the weight of said resin. The related art also does not teach a relationship between the temperature at which alcohol is removed in the aminopolyamide resin-making process and the wet strengthening ability of the aminopolyamide-epichlorohydrin resin made from the aminopolyamide resin. The related art also does not teach a relationship between the epichlorohydrin-aminopolyamide reaction time and very low total organically bound chlorine content in processes to make wet strength resins compositions comprised of an aminopolyamide-epichlorohydrin acid salt resins.